The strength properties of metastable austenitic steels can be improved by cryodeformation by their deformation below their respective martensite transformation temperature Md or Ms. Md is then the temperature above which a martensitic transformation does not take place even during deformation; Ms, on the other hand, is the temperature below which, even without deformation, the martensite formation begins. Such a process for the improvement of the strength properties of austenitic steels is also known from German DE-PS No. 26 54 702.
Since, in particular, the Ms temperatures are very low, the preferred employed cooling medium is liquid nitrogen which can be used to cool the steels if so desired to -196.degree. C.
The application of this process for the production of high strength pressure containers is known, moreover, from German DE-OS No. 1 452 533. The simultaneous use of liquid nitrogen as a cooling medium and pressure medium is then preferred. In this case, the container to be deformed is filled with liquid nitrogen and, by means of an appropriate cryopump or by gas pressurization, is brought to the high pressure required for the deformation. The use of a pressure medium which is different from the cooling medium is also mentioned but appears to be too expensive, for example, in the form of explosion deformation or undesirable condensations from the pressure medium can be expected, possibly a freezing up of the pressure medium with excessive cold extraction from the container wall.
In practice, however, the simultaneous application of liquid nitrogen as cooling and pressure medium has resulted in a number of disadvantages and problems.
When the pressure is transferred to the container wall via the liquid nitrogen as medium, the use of expensive heat-insulated apparatus such as cryopumps, insulated pipelines and cryocontainers is required. Gas pockets either produced on purpose or formed unavoidably in the container or its feed lines increase the safety risk if the container should fail during the cryostretching process. In addition, the liquid nitrogen per se at the relatively high, required stretching presusres (several 100 bar) also has a marked compressibility which in the case of failure clearly increases the released energy. Expensive safety devices which inhibit the industrial application of the process are, therefore, required for cryostretching.